This invention relates to a radiation curable adhesive composition used for surface protective films.
The surface protective films are used for surface protection of metallic plates such as stainless steel plates, aluminum plates and the like, coated metallic plates, resin plates, glass, etc. A characteristic fundamentally required for the films is that when the films are peeled off after being attached to these adherends, the adhesive does not transfer and adhere to the adherends. In order to maintain this property of not transferring and adhering to the adherends, much consideration is required because the surface protective films are often exposed under severe conditions such as high temperatures, exposure to outdoor circumstances and the like between the step of attaching and the step of peeling off. Therefore, as adhesives used in general, there are used those produced by crosslinking a polymer having relatively high molecular weight and viscosity to such a degree that the polymer does not lose its adhesive properties thereby enhancing the cohesion.
On the other hand, as the polymer there is usually used natural rubber, synthetic rubber or the like which should be coated on a substrate in the state of a solution prepared by dissolving it in an organic solvent. However, the production of solventless adhesives is noted from the viewpoint of prevention of environmental pollution, safety, saving resources and saving energy in recent years. As a means to produce a nonsolvent adhesive, radiation curable adhesives utilizing liquid oligomers are particularly noted.
Accordingly, the present inventors have developed a radiation curable adhesive having both cohesion and adhesive properties by adding a chain transfer agent while controlling the amount of unsaturated double bonds in a liquid oligomer having one or more radically crosslinkable unsaturated bonds in its molecule in order to crosslink the oligomer appropriately to enhance the cohesion, and have previously proposed the same (Japanese Patent Appln Kokai (Laid-Open) No. 10667/82).
However, such a radiation curable adhesive was disadvantageous in that since it is used under severe conditions such as high temperatures, outdoor circumstances and the like as mentioned above, its adhesiveness is deteriorated thereby cause the adhesive to transfer and adhere to the adherends.